1. Field of the Invention
This invention relates to a control circuit for a noisereducing signal transmission system of a type suitable for a tape recorder or the like and, in particular, it relates to a circuit with good transient response to suppress or eliminate noise superimposed on a signal between the recording and playback thereof.
2. The Prior Art
In the system described in a copending application Ser. No. 274,667, filed July 24, 1972, and now abandoned, it is stated that electrical information signals are, as a general matter, subject to having noise signals superimposed on them as these information signals are transmitted through a system or a series of systems from the point of generation to the point of reproduction. The term "system" is used to designate any means that may affect the passage of the signals and may be simple or complex. Various techniques have been proposed heretofore to combat the effects of such noise signals and particularly to combat noise introduced by the recording medium and apparatus in the case of systems in which the signals are recorded on magnetic tape or other recording media. Such noise is not uniformly distributed throughout the information signal frequency range, and it is possible to reduce the effect of this noise by controlling the frequency response characteristics of the system. However, it is desirable to retain a proper overall frequency response characteristic for the information signal, which means that any enhancement of the signal in one part of the system should be compensated by a reduction of the signal amplitude in another part of the system.
In the system described in the said copending application an output circuit of a signal amplifier is connected to a variable filter, that is, a filter having a variable frequency response characteristic that can be changed on a dynamic basis. The amplitude of the signals from the amplifier is modified by the variable filter in accordance with the instantaneous frequency response of the filter. The filter has a frequency response that attentuates one band of frequencies relative to another, i.e., low frequency signals relative to high frequency signals, within the complete range of the information signals. The filter is so arranged that the attenuation of signals at the low frequency end of the band is relatively constant, and the attenuation of the high frequency signals is also relatively constant, but between the low frequency and high frequency signals is a transition range in which the attenuation varies between the upper and lower limits. Furthermore, a control circuit is connected to the filter and is also connected to receive information signals to control the characteristics of the filter in such a way that the transition range can be shifted toward the high frequency end of the overall band or toward the low frequency end, depending on the amplitude of the information signals.
When such a system is to be used to record the information signals on a recording medium, such as magnetic tape, signals that have passed through the amplifier and the variable filter are made available at a system output circuit, such as a recording transducer. A negative feedback circuit may be connected from an output circuit to an input circuit of the amplifier when the system is part of a recorder.
On the other hand, if the amplifier and variable filter with its control circuit are to be used in a reproducing system, the output of the variable filter is connected back to an input circuit of the amplifier so that the variable filter is part of a negative feedback loop. In that case the signals that are more attenuated by the filter, will provide less negative feedback for the amplifier and thus will result in a higher output amplitude in the output circuit of the amplifier than those signals that are attenuated less by the filter. The output circuit of the amplifier arranged in this manner, may then be connected to a loud speaker or any other desired further circuit or load. These circuit components can be incorporated into a single device, such as a device for recording signals on tape and playing such signals back by providing a switch between the output of the filter and an input circuit of the amplifier. When this switch is closed, the signals from the filter are fed back to the amplifier; when it is open, these signals are not fed back to the input of the amplifier but may be applied to a recording head.
In particular, for reducing hiss generated by magnetic tape in a tape recorder, the variable filter is arranged so that when the information signal is in the high frequency portion of the overall band and at the same time has a relatively low amplitude, it will be amplified more than another signal of equal amplitude at the low frequency end of the overall frequency band. At an intermediate range of frequencies, the amplification will be dependent upon the precise frequency and will be between the maximum amplification of the high frequency signals and the minimum amplification of the low frequency signals. As the incoming signal increases in level, the transition band shifts so that the amplification of signals within the transition band will be reduced. In a reproducing system according to the present invention, the amplification of a high frequency, low level signal will be less than the amplification of a low frequency, low level signal. The transition band will be the same as in the recording system. Thus, the use of the same components for both recording and reproduction produces equal and opposite effects on the information signals.
It is desirable that the frequency response of the filter be controlled in response to high frequency, or transient, portions of the information signals as well as to low frequency portions. However, in the control amplifier of the aforesaid copending application, there is a smoothing filter to smooth out sharp variations in signal level. This makes the control amplifier non-responsive to transient signals. While the transient signals could be connected around or in parallel with the filtered signals, they must also have the same polarity as the filtered signals.
It is one of the objects of the present invention to provide a control circuit that not only includes means for rectifying and smoothing an incoming signal but also means for adding rapidly changing portions and doing so in the same polarity as the filtered signals, no matter which polarity the rapidly changing signals have.